One of the basic rules of English word construction is that a compound word takes its basic meaning and grammatical character from its final component. A housecat is a kind of cat, but a cathouse is a kind of house. Milk chocolate is a kind of chocolate, but chocolate milk is a kind of milk. (The latter two examples are considered compound words even though they are written with a space between the components.)

However, there seems to be a small but significant set of words which are exceptions to this rule. For example, killjoy ought by all rights to be a kind of joy. It ought to mean something like “the joy of the kill” — the feeling that makes you want to high-five your buddies after blowing the brains out of an eland. Iris Murdoch somewhere coins the word snowjoy (with reference to the emotions of dogs in winter), and it is instantly understandable. Hemingway would have been equally understandable if he had written of the killjoy of a toreador — but instead killjoy breaks the rules and means “one who kills joy.” Likewise, you ought to be able to say “He scoffed at the king and was jailed for violation of the scofflaw” — but in fact scofflaw follows the same pattern as killjoy and means “one who scoffs at the law.”

The other exceptions I’ve been able to think of all follow the same pattern: the structure is verb+noun, and the meaning is “one that [verbs] [noun].” Here’s my list so far: